The present invention relates to improvements in mechanisms for preparing pulp for papermaking machines, and more particularly to an apparatus for slicing wood chips.
In the paper industry, wood pulp is made by subjecting wood chips to a chemical digestion process wherein the compounds and chemical systems holding the fibers together, such as lignin, are dissolved to thereby liberate the individual wood fibers which are then diluted with water and introduced into a papermaking machine to make paper or paperboard products. The preparation of the wood chips is a critical part of the process in that, ideally, the wood chips should be of uniform thickness so that the chemicals penetrate the chips equally during the digestion process. If not properly formed, the chips may not be sufficiently penetrated to liberate the wood fibers, or if the chips are too thin, the chemicals may penetrate the chips too rapidly or for a longer time than necessary to liberate the individual fibers. The fibers themselves may be deleteriously weakened, or shortened or both.
The thickness of the individual wood chips is defined in the direction extending radially relative to the longitudinal axis of the log. Control of cutting the chips is difficult since the chips are sometimes gouged or broken out in chunks as a result of knots and compression. To ensure consistent chip size, screening systems are used to separate out oversize chips, which are directed to apparatus for reducing the size of the chips. Commercial type wood rechippers take various forms, and one successful form has been a rotary rechipper or chip slicer having a drum in which a rotor with vanes or anvils carries the wood in a rotary path to force the wood against knives circumferentially placed around the rotor.
A typical mechanism for slicing or rechipping wood chips is shown and described in U.S. Pat. No. 4,858,834. In a mechanism of this type, despite care taken in the feeding of the wood chips, foreign elements such as tramp iron may accompany the infeed of wood which is to be rechipped. When such tramp iron is caught by the rotor and carried against a knife, the rotor may be immediately forcibly stalled. With substantial input power, the chipper can be severely damaged and torn apart if power input continues and the rotor is not allowed to stall. Eventually, breakage will have to occur in different parts of the drive train as well as in the chipper if no provision for such unforeseen stalling is made. One attempt at providing for malfunctioning due to tramp iron and other failures which cause stalling of the rotor is to provide a hydraulic coupling between the drive and the rotor. This structure has not worked satisfactorily, and substantial equipment damages has resulted when the hydraulic coupling has not released, causing major down-time for customers with manufacturing and repair costs.
It is accordingly an object of the present invention to provide an improved chip slicer for use in a pulp preparation procedure which is capable of more continued satisfactory operation and the avoidance of costly equipment damage due to the intake of foreign objects into the slicer.
A further object of the invention is to provide an improved drive train arrangement for a chip slicer wherein the drive train arrangement takes care of malfunctioning and stalling of the chip slicer.
A further object of the invention is to provide an improved safety drive for a chip slicer wherein stalling of the rotor will be accommodated, and the drive train is constructed in a simplified and improved manner minimizing damages to the equipment with stalling, and permitting rapid repair with a minimum of down-time and minimum of cost.